Posted on another forum:
Yesterday I bought a .38-55 H&R "target" rifle. They have changed the primitive weapons rules here in Mississippi. Now you don't need a muzzle loader anymore, you can use a metallic cartridge rifle as long as it is breech loaded, has an exposed hammer, is .38 caliber or over and is an original or reproduction of a pre-1900's firearm. The H&R fit all the above criteria and was least expensive option out there.
Need a little help picking a bullet for my new .38-55 . I bought a box of Winchester .38-55's with the rifle and stuck one of the bullets in at the muzzle and there was so much space it rattled. Next I got out a box of .375 bullets I use to reload for my .375 H&H and dropped one down the barrel, it slid out the other end without any resistance!
I slugged the barrel this morning (just the muzzle end) and it mics out at .380 Given that, what bullet would you choose?
Gotta love those 38-55 dimensions.
His problem is common. Most of the old black powder cartridges were originally loaded with soft, undersize bullets. The undersized bullets chambered reliably in a fouled barrel, and black powder obturated the soft bullet to fit the barrel.
You would think that manufacturers would revise the outdated dimensions to be compatible with modern 375 bullets and smokeless powder, but no, that would be way too logical.
Here's the rub -- the chamber dimensions were also designed for undersize bullets. If you try loading a 0.381" bullet, you may find that the diameter of the cartridge neck is too big to fit in the chamber. Maybe, maybe not, it varies from rifle to rifle.
To answer you question, shoot 0.381" if they will chamber. Otherwise, do like grandpa did and shoot pure lead or 30:1 alloy and hope it will obturate.
I would not buy a 38-55 for that reason. Nothing inherently wrong with the cartridge, but why fight the goofy dimensional problems?